Gate valves are used to control the flow of various fluids carried inside conduits or pipes. A gate valve typically includes a main body structure with a fluid passageway positioned between and coupling a pair of axially aligned conduits that are bolted to opposite sides of the main body structure. A flat, fluid-impermeable gate is slidable through a slot in the main body structure to selectively open or close the passageway and hence the valve. In some gate valves the absence of the gate or a portion of it from the passageway opens the gate valve. In other gate valves, referred to as O-port gate valves, the gate includes a fluid impermeable portion and an apertured portion or "O-port." These valves are opened when the O-port, or a portion of it, is aligned with the fluid passageway.
Gate valves that are used to control the flow of fluids typically include a separate annular gate seal that is positioned between the gate and the main body structure to prevent the fluid from leaking. In an O-port gate valve, the annular gate seals remain in contact with the gate; either the impermeable portion or the portion surrounding the O-port. In other gate valves, the sleeve units compressibly engage each other when the valve is open and engage opposite sides of the gate when the valve is closed. Despite the use of annular gate seals, fluid leakage is a common problem with gate valves and can occur from the valve to the surrounding environment, across a closed valve, or from the passageway into interior portions of the valve.
Annular gate seals have been made from many different materials and in many different configurations but generally suffer from common disadvantages. Annular gate seals typically bear against the gate or each other under significant pressure. This pressure is necessary to maintain a seal, whether the gate valve is open or closed. A consequence of this pressure is that the seal is subjected to significant shearing forces as the gate is moved to open or close the valve. The shearing forces can damage or displace the seal, resulting in leakage.
Gate valves are commonly used in industrial applications (e.g., mining, wood pulp processing, paper manufacturing) in which the fluids or slurries carried by the valve can be particularly abrasive or chemically harsh due to temperature, pressure, or acidity. In addition, some fluids and the solids carried by some fluids (e.g., in slurries) can clog, coat, or damage many conventional seals. Another complication with conventional gate valves is that the slot through which the gate moves to open or close the gate is deeper than the gate is thick. This forms a cavity that prevents the gate from seizing to the interior surfaces of the valve. In many industrial environments, material that leaks into the valve body can harden to a solid mass within the cavity, making it extremely difficult or impossible to operate the valve.
Furthermore, the annular gate seals of conventional seals typically are molded and require a different mold for each size of valve. (Gate valves are sized according to the diameter of the conduit connected to the valve.) In manufacturing valves in a range of sizes, the mold for each size of seal can pose a significant manufacturing expense. The expense can be prohibitive if the volume of valves and seals in a particular size is relatively low and insufficient to recover the cost of the molds.
The present invention includes a gate valve having a pair of spaced-apart body halves with a slot positioned between them. The slot extends transverse, preferably perpendicular, to a fluid passageway. In one embodiment, an O-port gate is moveable within the slot and includes a pair of opposed major surfaces. The O-port gate includes a fluid impermeable portion and an apertured portion. A pair of opposed, plastic interior plates face each other and contact the opposed major surfaces of the O-port gate to form a cavityless gate valve.
The contact between the plastic interior plates and the major surfaces of the O-port gate form a sheet seal that effectively prevents leakage between the main body structure and the gate, thereby eliminating the need for a conventional annular gate seal. An advantage of the sheet seal is that it does not require the significant sealing pressures needed for localized annular gate seals, thereby eliminating the shearing forces that are so damaging to seals. In one embodiment, the sheet seal extends over substantially the entire slot. The plastic interior plates, formed of high-density polyethylene (HDPE), for example, have a lubricity that prevents the gate from seizing to them.
Another advantage of the plastic interior plates is that they eliminate the cavity of conventional gate valves. Elimination of the cavity means that leakage of certain industrial materials cannot fill a cavity, harden, and block operation of the valve. This provides a significant improvement in valve reliability in many industrial environments. Finally, elimination of the annular gate seal eliminates the cost of forming a seal mold for each size of valve.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.